It is known that a microwave signal of a steady state, which responds to a direct current, can be generated by using spin transfer effect which occurs in a magnetic multilayer film of nanometer scale (for example, see S. I. Kiselev et al. “Microwave oscillations of a nanomagnet driven by a spin-polarized current” Nature 425, 380 (2003)). The origin of the microwave signal is magnetization oscillation of a magnetization oscillation part in a magnetic multilayer film. In an experiment, in a current-perpendicular-to-plane (CPP) giant-magnetoresistive (GMR) effect film and a magnetic tunnel junction (MTJ) film, when the current density exceeds the order of 107 A/cm2, steady magnetization oscillation of high frequency (GHz) is detected.
Microwave generators using spin transfer effect generated in a magnetic multilayer film are called spin transfer oscillators, magnetic oscillators, and spin transfer oscillators. By a remarkably-advanced fine processing technology, it has become possible to process a CPP-GMR film and a magnetic tunnel junction film in a submicron size of about 100 nm×100 nm. Magnetic oscillators are expected to be applied to minute microwave sources and resonators, and have been actively researched as a research of spintronics. The frequency of a microwave signal generated from a magnetic oscillator depends on a current, and a magnetic field which acts on magnetization of a magnetization oscillation part in a magnetization multilayer film. In particular, by using its magnetic field dependence that the magnetization oscillation frequency changes according to the magnetic field, it has been proposed to apply magnetic oscillators to magnetic sensors for an HDD which replace a GMR head and a TMR head (for example, see JP-A 2006-286855 (KOKAI)). When a magnetic oscillator is used as a magnetic sensor for an HDD, the magnetic field of the HDD medium is sensed by detecting change in frequency caused by the magnetic field.
Conventional magnetic oscillators have a structure in which a microwave signal caused by oscillation of magnetization in a magnetoresistive element having a ferromagnetic multilayer film is taken out. The magnetoresistive element has a three-layer structure including a magnetization free layer, a spacer layer, and a magnetization pinned layer, as a basic structure. When a direct current I flows through the magnetoresistive element by a power supply, the magnetization in the magnetization free layer is oscillated by a spin transfer effect between the magnetization free layer and the magnetization pinned layer, and an angle θ between the magnetization of the magnetization free layer and the magnetization of the magnetization pinned layer changes from moment to moment. With the change of the relative angle θ, the element resistance changes from moment to moment mainly by magnetoresistive effect, and therefore an alternating-current component of the voltage is produced. By extracting the alternating-current component of the voltage by a bias tee, a microwave signal is obtained.
A direct current I generated by a power source is not a desired value, but must be a current value which exceeds a threshold current value Ic that depends on the structure of the magnetoresistive element module including a ferromagnetic multilayer film and the surrounding magnetic field environment. Only when I>Ic is satisfied, magnetization oscillation is induced in the magnetization free layer by the spin transfer effect. The value of the threshold current Ic is determined by a cross section of the magnetoresistive element and a threshold current density value. It is known that the threshold current density value is about 107 A/cm2.
In the meantime, there is a quality (Q) factor as a quantity which indicates a character of the oscillator. As an example of a Q-factor, there is mentioned an oscillating circuit which uses a crystal oscillator as a resonator. It is known that crystal oscillators have a high Q-factor of the order of 106. An oscillating circuit which uses a crystal oscillator as a resonator achieves a Q-factor of the order of 103 to 104, and obtains stable oscillation. The Q-factor is a dimensionless quantity which is defined as follows, and a large Q-factor means that oscillation is stable.
  Q  =            energy      ⁢                          ⁢      stored      ⁢                          ⁢      in      ⁢                          ⁢      1      ⁢                          ⁢      period              power      ⁢                          ⁢      loss      ⁢                          ⁢      consumed      ⁢                          ⁢      in      ⁢                          ⁢      1      ⁢                          ⁢      period      ⁢                          ⁢              (                  dissipated          ⁢                                          ⁢          energy                )            
Oscillated state is often recognized by a frequency spectrum thereof, and in such a case the Q-factor is defined by Q=f0/Δf. The symbol f0 represents an oscillation frequency, and the symbol Δf represents a full width at half maximum of an oscillation peak of the frequency spectrum.
A magnetic oscillator realized by a CPP-GMR film (hereinafter referred to as a “GMR oscillator”) is obtained when a spacer layer of the magnetoresistive element is formed of a non-magnetic metal layer such as Cu. It has been known from experiment that oscillation of Q≈(10 GHz/1 MHz) 104 is obtained by a GMR oscillator (for example, see W. H. Rippard et al. “Current-driven microwave dynamics in magnetic point contacts as a function of applied field angle” Physical Review B 70, 100406 (R) (2004)). Specifically, GMR oscillators have performance which is greater than or equal to oscillating circuits which use a crystal oscillator as a resonator, with respect to the Q-factor. The reason why GMR oscillators can achieve a high Q-factor is that a large current can flow through GMR oscillators that are artificial metal lattices, all of which are formed of metal material. It is known that a full width at half maximum Δf of the frequency spectrum is generally in inverse proportion to the square of current I (that is, Δf∝1/I2). The value of Δf becomes extremely small by flowing a large current, and thus a high Q-factor can be achieved. A high Q-factor is an advantage of GMR oscillators. GMR oscillators have, however, a disadvantage that a single GMR oscillator outputs a weak electric power of the order of nanowatts (nW) at most, which is far from a practical electric power level of microwatts (μW) and is not desirable for application. The reason why a GMR oscillator outputs a weak electric power of the order of nanowatt is that GMR oscillators have a small magnetoresistive (MR) ratio of several percent at most. A structure of increasing an output power by arranging GMR oscillators in an array has been proposed (for example, see S. Kaka et al. “Mutual phase-locking of microwave spin torque nano-oscillators” Nature 437, 389 (2005)). In the case of arranging GMR oscillators in an array, however, it is necessary to arrange at least dozens of GMR oscillators in an array and synchronize all the oscillators with each other, to increase the output power to a microwatt level. Therefore, it is difficult to manufacture the magnetic oscillator.
On the other hand, magnetic oscillators achieved by a magnetic tunnel junction film (hereinafter referred to as “TMR oscillators”) are obtained when a tunnel barrier is used as the spacer layer. In recent years, high-quality magnetic tunnel junction films which have low resistance and a high MR ratio have been developed, and expected to be applied to spin injection magnetic random access memories (spin-RAM). In particular, it has been known by experiments that the MR ratio in a TMR (MgO-TMR) film which has a magnesium oxide (MgO) barrier is several hundred percent or more. TMR oscillators can obtain large oscillation power P since they have a high MR ratio. The oscillation power generated by magnetic oscillators using an MgO-TMR film is actually coming near practical microwatt electric power level, and the maximum power level which has been reported at present is 0.16 μW. It is impossible, however, to cause a large current to flow through magnetic oscillators using a magnetic tunnel junction film such as an MgO-TMR film, unlike GMR oscillators, due to the problem of insulation breakage by a tunnel barrier, and thus it is difficult for the oscillators to realize a high Q-factor.
There are many cases where magnetization oscillation cannot be excited in the first place in TMR oscillators. This is also due to insulation breakdown of the tunnel barrier. This is because there are many cases where insulation breakdown is caused by a current which is smaller than the threshold current Ic, although magnetization oscillation is excited in the free layer by the spin transfer effect only when I>Ic is satisfied as described above.
JP-A 2009-194070(KOKAI) discloses a complex magnetic oscillator which is obtained by magnetostatic-coupling an oscillation driving module formed of a GMR oscillator with an output module formed of a TMR oscillator, with good use of characters of GMR oscillators and TMR oscillators. It is necessary, however, to manufacture the two oscillators very close to each other, that is, 300 nm or less, to perform magnetostatic coupling, and thus the manufacturing process is difficult both in a planar structure and a layered structure.
As described above, each of GMR oscillators and TMR oscillators have merits and demerits. The merit of GMR oscillators is a high Q-factor, and the demerit thereof is small oscillation power. The merit of TMR oscillators is large oscillation power, that is, high output power, and the demerit thereof is a low Q-factor.
Therefore, it is required for magnetic oscillators to have the advantages of GMR oscillators and TMR oscillators, that is, a high Q-factor and high output power.